<html>
<head>
  <script>
  addEventListener('load', function() {
    document.getElementsByTagName('form')[0].submit();
  });
  </script>
</head>
<body>
  <form action="file_empty.html" method="POST">
    <input type="text" name="postvalue" value="I am a test string!!" />
    <input type="submit" />
  </form>
</body>
</html>
